How do vaccines work?

DEAR DOCTOR K:

You’ve written many columns about vaccines. Can you explain how they work?

DEAR READER:

A vaccine prompts your immune system to build immunity against a particular germ. It mimics what would happen naturally if the germ entered your body. In order to understand how vaccines work, though, it helps to understand how your body’s immune system works.

When disease germs enter your body, they grow and multiply. Immune system cells recognize these germs as “not you.” One way your immune system protects you against infection is by making proteins called antibodies. The antibodies attach themselves to the germs. This signal causes other cells of the immune system to kill the germs.

When the immune system fights an infection for the first time, it creates “memory cells.” If you are ever infected by that germ again, the memory cells recognize it and initiate a rapid attack on it.

So that’s how your immune system responds to a real infection. Vaccines also provoke an immune response. But there is one essential difference — there’s no harmful germ involved. Instead, the vaccine contains a recognizable but harmless version of the germ. When you’re vaccinated, your immune system is fooled into thinking that the germ has invaded your body. Your immune system responds by making antibodies and memory cells — but without making you sick first.

There are two main types of vaccines:

LIVE ATTENUATED VACCINES contain weakened forms of a specific disease-causing germ. The weakened virus multiplies rapidly. This prompts a full-scale immune response. Not everyone can safely get a live attenuated vaccine. People with weak immune systems, for example, usually shouldn’t get a live attenuated vaccine. That’s because there is a small chance it could cause a mild version of the disease. What for most people would be a harmless, weak germ is for them a potentially harmful germ.

INACTIVATED VACCINES are made by killing disease-causing germs with heat, chemicals or radiation. There is no chance that an inactivated vaccine can cause the disease it is intended to prevent. A killed virus can’t multiply. As a result, the antibody response is limited compared to a live attenuated vaccine. Some inactivated vaccines require one or more “booster” shots to maintain immunity.

Some vaccines, like the flu shot, are available in both live and inactivated versions. Vaccines for other diseases, such as the measles, mumps and rubella (MMR) vaccine, are available only in the live attenuated form.

In recent years, new types of vaccines have been developed. Whereas most vaccines use proteins from a germ, DNA vaccines include the sequence of the germ’s DNA that directs cells to make a particular protein. Such vaccines may have a longer-lasting effect than traditional vaccines, and are relatively inexpensive to use in developing nations.

An even more recent type of vaccine uses a harmless virus to carry DNA that directs the production of specific antibodies against HIV. Mice whose immune cells were replaced with human immune cells were given the vaccine, and then exposed to HIV. The antibodies the mice were making against HIV protected them from infection by HIV.

Vaccines, one of the most important medical technologies ever developed, are taking new forms, with very promising results.